The invention relates to generally regenerative type burners for heating a furnace, and more particularly to regenerative burners with minimized NOx formation in the ultimate combustion effluents. This invention is related to co-pending application Ser. No. 168,892 entitled "Low NOx Regenerative Burner" filed Mar. 16, 1988, the contents of which are incorporated by reference herein.
Regenerative-type burners for furnaces are well-known in the art in varied forms and designs, but they share the common feature whereby heat storage units are provided to withdraw and store heat from hot combustion effluents known as flue gas, with subsequent transfer of the heat to preheat incoming combustion air. The earliest regenerative-type furnaces were symmetrical arrangements having both burner(s) and heat storage units (often solid structural arrays of "checker chamber" bricks) in place on each of two sides of the furnace. Firing of such a regenerative furnace began with the burner(s) on one side, with concomitant storage of the heat present in the combustion effluents by the heat storage units on the second side. After optimal heating of the heat storage units, or the "checker chamber," the air flow in the furnace was reversed to draw combustion air in through the checker chamber, thus preheating the combustion air. Ducts in the checker chamber thus alternately conveyed combustion products and combustion air, and the burners functioned alternately as burners and as flues.
Modern regenerative systems do not involve complete symmetrical furnaces but instead include specialized regenerative burners employed, typically, in pairs. Each of the paired regenerative burners is equipped with heat storage units, ordinarily in the form of compact regenerative beds, through which combustion air passes en route to the burner. Because the burners are employed in pairs, one burner is fired at a time while the other functions as a flue and heat storage bed. Then every 20-120 seconds or so, flow in the furnace is reversed and the burners "exchange" functions, that is, the first-fired burner becomes the flue gas exhaust/heat storage bed as the second burner fires. A system exemplary of one paired burner arrangement is found in U.S. Pat. No. 4,522,588.
A persistent problem with regenerative systems involves the extremely high NOx concentrations inevitably present in the combustion effluents, produced as a result of the extremely high air preheats and flame temperatures, as well as through fuel bound nitrogen. As a result, regenerative systems which historically enjoyed industry-wide acceptance now cannot meet the emissions standards in an ever-increasing number of localities and/or process conditions. There is a need for low NOx burner concepts which can be broadly adapted to the specific applications by altering the flame temperature to meet NOx emission requirements. A need therefore persists for regenerative burner systems which provide the heat-regenerative function of prior art systems yet provide for significant NOx reduction.